1. Field of the Invention
The present invention relates to throttle control devices that have a motor and a reduction gear mechanism that is driven by the motor in order to rotate a throttle valve for controlling a flow rate of intake air supplied to an engine, e.g., an internal combustion engine of an automobile.
2. Description of the Related Art
Japanese Laid-Open Patent Publication No. 6-264777 teaches a known throttle control device. As shown in FIG. 14, the known throttle control device has a motor 92 and a reduction gear mechanism 94 that is driven by the motor 92 in order to rotate a throttle valve 96 for controlling a flow rate of intake air. A movable section 102 of a throttle sensor 100 is coaxially mounted on one end of a rotary shaft 92s of the motor 92. The movable section 102 has a disk-like configuration including concave and convex portions. The concave and convex portions are formed on the outer periphery of the movable section 102 and are arranged at predetermined intervals in the circumferential direction. A fixed sensing section 104 of the throttle sensor 100 is mounted on the throttle body 91 and is adapted to detect the concavity or the convexity of the movable section 102.
Therefore, as the movable section 102 of the throttle sensor 100 rotates together with the rotary shaft 92s of the motor 92, the fixed sensing section 104 of the throttle sensor 100 detects the concave or convex portions of the movable section 102 in order to count the number of concave or convex portions moving past the sensing section, so that the rotational angle of the motor 92 and consequently the degree of opening of the throttle valve 96 can be determined. Because the rotational angle of the throttle valve 96 is determined based upon the rotational angle of the motor 92, the accuracy of the measurement of the rotational angle of the throttle valve 92 can be improved in comparison with an arrangement in which the rotational angle of a throttle valve is directly detected.
Here, in order to provide a level of precision for the measurement, the outer diameter of the movable section 102 is set to be substantially equal to the outer diameter of the motor 92.
However, the throttle sensor 100 of the known throttle control device is configured to detect the concave or convex portions formed on the outer periphery of the disk-like movable section 102 and to count the number of the concave or convex portions in order to obtain the rotational angle of the throttle valve 92. Therefore, the throttle sensor 100 must have a large size in a diametrical direction to accommodate the number of concave and convex portions required for accuracy. For this reason, the space for accommodating the motor 92 having the throttle sensor 100 must be large in size in a diametrical direction in comparison with a space required for accommodating only the motor 92. Therefore, a problem has been that the throttle body 91 must also have a relatively large size.